To improve thermal efficiency of gasoline internal combustion engines, dilute combustion—using either air or re-circulated exhaust gas—is known to give enhanced thermal efficiency and low NOx emissions. However, there is a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the dilution limit include 1) improving ignitability of the mixture by enhancing ignition and fuel preparation, 2) increasing the flame speed by introducing charge motion and turbulence, and 3) operating the engine under controlled auto-ignition combustion.
The controlled auto-ignition process is sometimes called the Homogeneous Charge Compression Ignition (HCCI) process. In this process, a mixture of combusted gases, air, and fuel is created and auto-ignition is initiated simultaneously from many ignition sites within the mixture during compression, resulting in very stable power output and high thermal efficiency. The combustion is highly diluted and uniformly distributed throughout the charge, so that the burned gas temperatures and hence NOx emissions are substantially lower than those of traditional spark ignition engines based on a propagating flame front and diesel engines based on an attached diffusion flame. In both spark ignition and diesel engines, the burned gas temperatures are highly heterogeneous within the mixture with very high local temperatures creating high NOx emissions.
Engines operating under controlled auto-ignition combustion have been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burned gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture.
In four-stroke engines with traditional valve means, the residual content is low and controlled auto-ignition at part load is difficult to achieve. Methods to induce controlled auto-ignition at low and part loads include: 1) intake air heating, 2) variable compression ratio, and 3) blending gasoline with ignition promoters to create a more easily ignitable mixture than gasoline. In all the above methods, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow.
Engines operating under controlled auto-ignition combustion have been demonstrated in four-stroke gasoline engines using variable valve actuation with unconventional valve means. The following are descriptions involve valve strategies in which a high proportion of residual combustion products from a previous combustion cycle is retained to provide the necessary conditions for auto-ignition in a highly diluted mixture. The range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is greatly expanded using a conventional compression ratio.
In one instance, a four-stroke internal combustion engine is reported to provide for auto ignition by controlling the motion of the intake and exhaust valves of a combustion chamber to ensure that a fuel/air charge is mixed with combusted gases to generate conditions suitable for auto-ignition. The described engine has a mechanically cam-actuated exhaust valve that is closed earlier in the exhaust stroke than normal four-stroke engines to trap combusted gases for subsequent mixing with an intake of fuel and air mixture.
Another method is described of operating a four-stroke internal combustion engine in which combustion is achieved at least partially by an auto-ignition process. Flows of fuel/air charge and combusted gases are regulated by hydraulically controlled valve means in order to generate conditions in the combustion chamber suitable for auto-ignition operation.
The valve means used comprises an intake valve controlling the flow of fuel/air mixture into the combustion chamber from an inlet passage and an exhaust valve controlling exhaust combusted gases from the combustion chamber to an exhaust passage. The exhaust valve opens (EVO) at approximately 10 to 15 degrees before bottom dead center in the expansion stroke, and closes (EVC) during the exhaust stroke in a range of 90 to 45 degrees before top dead center. The intake valve is opened (IVO) later in the four-stroke cycle than usual in a normal four-stroke engine in the range of 45 to 90 degrees after top dead center during the intake stroke.
The early exhaust valve closing and late intake valve opening provide a negative valve overlap period (EVC-IVO) where both exhaust and intake valves are closed for trapping of combusted gas which later mixes with the inducted fuel/air charge during the intake stroke and thereby promotes the auto-ignition process. The intake valve is then closed (IVC) roughly 30 degrees after bottom dead center in the compression stroke. This is generally referred to as an exhaust re-compression valve strategy.
In another described method of operating a direct-injection gasoline four-stroke internal combustion engine, combustion is achieved at least partially by an auto-ignition process. Flows of air and combusted gases are regulated by a hydraulically controlled valve means. The fuel is delivered directly into the combustion chamber by a gasoline injector. The gasoline injector is said to inject fuel during either the intake stroke or the subsequent compression stroke in a single engine cycle.
Using exhaust re-compression valve strategy in conjunction with a gasoline direct injector having multiple injection capability during a single engine cycle, we and others have demonstrated that the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is greatly expanded using a conventional compression ratio. Further, we have also demonstrated that low load fuel economy and ability to start the engine cold are superior using exhaust re-compression valve strategy coupled with stratified ignition. However, torque fluctuation increases during idle and low load engine operations may be a concern. This is the subject of the present invention.